Membrane separation techniques are used in a food and medical field and a water treatment field. In recent years, as represented by application of a membrane separation technique in ethanol production using biomass, i.e., a membrane separation technique of water and ethanol, change of composition of a mixture has been conducted by separating a specific component from a mixture.
Regarding a separation operation of a liquid mixture using a membrane separation technique, in recent years, application to non-aqueous fields, for example, a petroleum refinery process and a petrochemical industry field has been studied (e.g., Patent Documents 1 to 3). For example, Patent Document 2 discloses a separation membrane used for changing a composition of a liquid mixture containing a paraffin-based hydrocarbon liquid and an olefin-based hydrocarbon liquid by a separation operation. Regarding membrane separation techniques, besides such separation of a liquid mixture containing hydrocarbon-based liquids, there has recently been disclosed a trial of application to separation of a liquid mixture containing a hydrocarbon-based liquid and an alcohol liquid for improvement in start-up performance, highly efficient combustion, and cleaning of an internal combustion engine in the fuel field (e.g., Patent Documents 4 to 6).
Patent Document 5 discloses a fuel supplier using a separation membrane using a membrane of an inorganic metal oxide and/or a membrane of a polymer compound to separate a mixed fuel obtained by adding an ethanol fuel to a hydrocarbon-based fuel into a hydrocarbon rich fuel and an ethanol rich fuel. Non-patent Document 1 has a statement that, in a production process of methyl tert-butyl ether (MTBE) or ethyl tert-butyl ether (ETBE) as an octane booster of gasoline, separation of methanol from MTBE or ethanol from ETBE as azeotropic mixtures by a separation membrane is studied.
As a separation membrane for a liquid mixture in a non-aqueous field as described above, a polymer membrane, a zeolite membrane, a carbon membrane, and the like are described. However, the polymer membrane and the zeolite membrane have a defect of low corrosion resistance against an organic solvent or an acid/alkali aqueous solution to deteriorate the performance upon use for a long period.
A carbon membrane is expected to application to separation of a liquid mixture in a non-aqueous field by utilizing the characteristics of a carbon membrane having excellent thermal resistance, acid resistance, and organic solvent resistance (Non-patent Document 2). However, the carbon membrane has a problem of low permeability with high separability or low performance with high permeability. Thinning of the membrane is mentioned as a method for improving the permeability. In this case, there is a problem of deterioration in separability because a defect is easily caused.
There is known a method in which, after a thermosetting resin is formed, it is carbonized and/or activated at 600 to 1100° C. in weak oxidizing atmosphere as a method for enhancing the permeation separability of the carbon membrane for the liquid mixture (Patent Document 7).
As a method for improving permeability of a carbon membrane other than Patent Document 7, there is known a treatment in which a carbon membrane is heated in the presence of oxidizing gas (heating oxidation treatment). For example, in Non-patent Document 3 (FIG. 3), permeability of carbon dioxide and nitrogen improved about 13 times and 40 times, respectively, though separability of carbon dioxide and nitrogen (ratio of permeation rate of carbon dioxide and nitrogen) of the membrane fell from about 30 to 10.